InterPro domain: IPR023123

Microtubules are polymers of tubulin, a dimer of two 55kDa subunits, designated alpha and beta [ 1 , 2 ]. Within the microtubule lattice, alpha-beta heterodimers associate in a head-to-tail fashion, giving rise to microtubule polarity. Fluorescent labelling studies have suggested that tubulin is oriented in microtubules with beta-tubulin toward the plus end [ 3 ].

For maximal rate and extent of polymerisation into microtubules, tubulin requires GTP. Two molecules of GTP are bound at different sites, termed N and E. At the E (Exchangeable) site, GTP is hydrolysed during incorporationinto the microtubule. Close to the E site is an invariant region rich in glycine residues, which is found in both chains and is thought to controlaccess of the nucleotide to its binding site [ 4 ].

Most species, excepting simple eukaryotes, express a variety of closely-related alpha- and beta-isotypes. A third family member, gamma tubulin, hasalso been identified in a number of species. Gamma tubulin is found at microtubule-organising centres, such as the spindle poles or the centrosome, suggesting that it is involved in minus-end nucleation of microtubule assembly [ 5 ]. Further eukaryotic tubulins (gamma, epsilon, zeta) that are restricted to certain lineages or species have been reported [ 6 ].

Bacterial and archaeal homologues of tubulin have been discovered. BtubA and BtubB, two bacterial homologues in the genus Prosthecobacter, have probably been derived by horizontal gene transfer [ 7 , 8 ].

This entry represents the extreme C-terminal structural domain of both alpha and beta tubulin. It forms a helix hairpin [ 9 ].

1. Molecular biology and genetics of tubulin. Annu. Rev. Biochem. 54, 331-65
2. Diversity among tubulin subunits: toward what functional end? Cell Motil. Cytoskeleton 16, 159-63
3. Localization of an exchangeable GTP binding site at the plus end of microtubules. Science 261, 1044-7
4. Tubulin sequence region beta 155-174 is involved in binding exchangeable guanosine triphosphate. J. Biol. Chem. 262, 15472-5
5. Gamma-tubulin: the hub of cellular microtubule assemblies. Bioessays 15, 637-43
6. Six subgroups and extensive recent duplications characterize the evolution of the eukaryotic tubulin protein family. Genome Biol Evol 6, 2274-88
7. Genes for the cytoskeletal protein tubulin in the bacterial genus Prosthecobacter. Proc. Natl. Acad. Sci. U.S.A. 99, 17049-54
8. Structure of bacterial tubulin BtubA/B: evidence for horizontal gene transfer. Proc. Natl. Acad. Sci. U.S.A. 102, 9170-5
9. The 4 A X-ray structure of a tubulin:stathmin-like domain complex. Cell 102, 809-16